In joint drainage systems, in which sewage and rain water are discharged into the same sewer, concentrators are used in order to avoid hydraulic overloading of the purifying plant in case of heavy rain and subsequently big amounts of water in the sewer. The simplest, but also the least effective concentrator, is an overfall construction. Such a construction comprises a reservoir with an outlet, an overfall edge and a delivery pipe, normally with a comparatively big cross-section, because it has to be dimensioned for peak loads. Therefore, the flow rate in the delivery pipe will typically be relatively small and suspended matter will sink towards the bottom of the pipe. Immediately before the overfall construction, the concentration of suspended matter will therefore normally be higher at the bottom than at the top. This effect decreases with increasing flow rate and on account of turbulence. No requirements are made today in overfall constructions with respect to how the delivery is to take place, and a problem is that the turbulence from the overfall edge may be so heavy that the amount of water flowing over the overfall edge may be nearly just as polluted as the amount of water let out through the cutoff outlet to the purifying plant.
Among those skilled in the art there is a general agreement that it serves no purpose to arrange bigger reservoir volumes in joint sewage systems to limit the amount of impurities delivered to the receiver. Thus, measurements have shown that the amount of suspended matter in the water supplied to the receiver is by and large the same irrespective of whether it is supplied through overflow or through a purifying plant which has been peak loaded for a long time, because a purifying plant with a protracted peak load or momentary peak load loses its effect due to the fact that the active sludge is washed away.